Electron impact (EI) ionization is widely employed for environmental analysis and technological control. Samples of interest are extracted from analyzed media, like food, soil or water. The extracts contain impurities of interest within rich chemical matrixes. The extracts are separated in time within single or two-dimensional gas chromatography (GC or GC×GC). A GC carrier gas, typically Helium, delivers the sample into an EI source for ionization by an electron beam. Electron energy is generally kept at 70 eV in order to obtain standard fragment spectra. Spectra are submitted for comparison with a library of standard EI spectra for identification of compounds.
Many applications demand analysis of ultra traces at high level of sensitivity (e.g., at least under 1 pg and preferably at 1 fg level) and with a high dynamic range (e.g., at least 1E+5 and desirably at 1E+9) between ultra traces and rich chemical matrix. Data with high resolving power is generally sought for reliable compound identification and for improving a ratio of signal to chemical noise.
Many GC-mass spectrometer systems employ quadrupole analyzers. Since EI spectra contain a multiplicity of peaks, it is generally necessary to use a scan mass analyzer over a wide mass range, which leads to inevitable ion losses, slows down spectra acquisition, and introduces skew in the shape of individual mass traces, distorting fragment intensity ratios. Since GC and in particular GC×GC separation provide short chromatographic peaks (e.g., under 50 ms in GC×GC case), a Time-of-flight mass spectrometer (TOF MS) is generally used for rapid acquisition of panoramic (full mass range) spectra.
To separate target compounds of close isobars of matrix ions and their low abundant isotopes it is desirable to employ high resolution mass spectrometers, such as multi-reflecting TOF MS (MR-TOF MS). As example, separating a pair of 13C from 12CH with 4.5 mDa mass difference for ions with mass 300 requires 75,000 resolution. For dioxin analysis the required resolution is 10,000 at 10% peak height and for some toxic benzo-furans resolution should exceed 18,000.
WO2012024468, incorporated herein by reference, describes a method of ion storage within an electron beam of so-called “closed” EI source followed by synchronized release into an orthogonal accelerator (OA) for ultra-sensitive analysis (LOD=1 fg) in MR-TOF at high resolution in excess of 30,000. However, the EI storage capacity tends to saturate by space charge when trace compounds are analyzed within concentrated matrices at momentarily sample fluxes in excess of 1 ng/sec, thus limiting the detectable threshold of relative sample concentration per matrix above 10 ppb (parts per billion). To bypass the limitation, a laborious sample preparation is required. Thus, a lower detection threshold is desired for analysis of highly toxic compounds such as pesticides in baby food or dioxins in environmental samples.